1. Field of the Invention
The present invention relates to a solid state imaging device including a photoelectric conversion element group made up of a large number of photodiodes, each of which converts incident light into an electric charge and accumulates the electric charge, and a charge transfer device connected to the photoelectric conversion element group. Further, the present invention relates to a method of driving the above solid state imaging device.
2. Description of the Prior Art
FIG. 1 shows an example of a solid state imaging device of the interline transfer type which includes a charge coupled device (hereinafter simply referred to as "CCD") and a photoelectric conversion element group made up of photodiodes. In FIG. 1, reference numeral 400 designates a light receiving part, 600 photoelectric conversion elements (that is, photodiodes) each capable of converting incident light into an electric charge and accumulating the electric charge, 101 to 10N vertical charge transfer means (that is, vertical CCD's) for transferring the electric charges accumulated in the photoelectric conversion elements, in vertical directions indicated by arrows, 201 a terminal applied with a transfer pulse signal .phi..sub.V1, and 202 a terminal applied with another transfer pulse signal .phi..sub.V2. The transfer pulse signals .phi..sub.V1 and .phi..sub.V2 are synchronized with the horizontal synchronizing signal of television signal.
A signal charge accumulated in a photoelectric conversion element 600 is transferred to one of the vertical CCD's 101 to 10N which is adjacent to the photoelectric conversion element, in a vertical retrace period by a switching pulse signal .phi..sub.V0 applied to a terminal 204. After having been transferred, the signal charge is transferred upwardly in the above one of the vertical CCD's 101 to 10N by the pulse signals .phi..sub.V1 and .phi..sub.V2.
Further, reference numeral 300 designates a horizontal charge transfer means (that is, a horizontal CCD) for transferring an electric charge in a horizontal direction by a horizontal pulse signal .phi..sub.H applied to a terminal 203. Signal charges which have been transferred from the vertical CCD's 101 to 10N to the horizontal CCD 300, are delivered, as a video signal, to the outside through an amplifier 500 and an output terminal 205, in one horizontal period.
In the imaging device of FIG. 1, a signal charge accumulated in a photoelectric conversion element 600 is transferred to one of the vertical CCD's 101 to 10N which is adjacent to the photoelectric conversion element, on the basis of the saturation characteristics of a switching MOS transistor 206 which is formed of an MOS field effect transistor, connected between the photoelectric convertion element and the above vertical CCD, and driven by the switching pulse signal .phi..sub.V0.
FIG. 2 shows the structure of the vertical CCD 10N of FIG. 1, viewed in the direction of the thickness thereof, accompanied with an equivalent circuit diagram. In FIG. 2, reference numeral 110 designates a semiconductor substrate, 111 gate electrodes formed of a first polysilicon layer, 112 gate electrodes formed of a second polysilicon layer, 113 a schematic potential level formed under the gate electrodes 111 and 112, and 114 a signal charge to be transferred by the potential level.
Each of the vertical CCD's 101 to 10N shown in FIG. 1 is driven by the two pulse signals .phi..sub.V1 and .phi..sub.V2 which are different in phase from each other, as shown in FIG. 2. Accordingly, it is required to operate each vertical CCD so that a stage 116 or 118 having no signal charge exists between two stages each having a signal charge. (In recent years, vertical CCDs are often driven by four pulse signals which are different in phase from each other. In this case, it is also required to drive the vertical CCD in such a manner that a stage having no signal charge exists between two stages each having a signal charge as in the driving method of FIG. 2.)
The signal charges accumulated in the photoelectric conversion elements 600 are transferred to the first to M-th stages of each of the vertical CCD's 101 to 10N by the switching pulse signal .phi..sub.V0. In the first field of television signal, however, signal charges from the photoelectric conversion elements in the first and second rows are collected in the first stages of the vertical CCD's 101 to 10N, and signal charges from the photoelectric conversion elements in the third and fourth rows are collected in the third stages of the vertical CCD's 101 to 10N. That is, signal charges from all the photoelectric conversion elements 600 in the light receiving part 400 are collected in odd-numbered stages of the vertical CCD's 101 to 10N, and even-numbered stages of the vertical CCD's 101 to 10N have no signal charge. The odd-numbered stages and even-numbered stages of a vertical CCD are used for transferring signal charges therein. In the horizontal retrace period of television signal, signal charges in a vertical CCD are transferred by two stages, and the sum of signal charges from two photoelectric conversion elements in adjacent rows, for example, in the first and second rows is transferred to the horizontal CCD 300, to be read out from the output terminal 205 in the horizontal effective scanning period which follows the above horizontal retrace period. Similarly, the sum of signal charges from two photoelectric conversion elements in the third and fourth rows, the sum of signal charges from two photoelectric conversion elements in the fifth and sixth rows, and so on are successively read out at intervals of one horizontal period.
While, in the second field, the sum of signal charges from two photoelectric conversion elements in the second and third rows, the sum of signal charges from two photoelectric conversion elements in the fourth and fifth rows, and so on are successively read out at intervals of one horizontal period, to carry out an interlaced scanning.
In the imaging device of FIG. 1, however, part of an electric charge which is generated beneath a photodiode 600 by incident light, may leak in a vertical CCD in a period when signal charges are transferred in the vertical CCD, and thus a smear may appear on a display screen in the form of a vertical, white belt. Further, the sum of signal charges from a pair of photodiodes in adjacent rows is always read out, that is, it is impossible to read out signal charges from all the photodiodes in the light receiving part 400 independently of each other. Accordingly, when a single chip color imaging device is formed of the device of FIG. 1, the resolution of the color imaging device is reduced, and a Moire pattern appears on a display screen. Hence, it is required to read out signal charges from all the photodiodes independently of each other at every field. In an improved imaging device capable of reducing the above-mentioned smear, a charge storage part for storing signal charges corresponding to one field of television signal (that is, (M.times.N)/2 signal charges) is provided between the light receiving part 400 and horizontal CCD 300 (where M indicates the number of rows in which the photodiodes are arranged, and N the number of photodiodes in one row). In this device, signal charges of all the photodiodes are transferred to the charge storage part in the horizontal retrace period, which is a very short period. (It is to be noted that, in the device of FIG. 1, signal charges of all the photodiodes are taken out from light receiving part 400 in a period corresponding to one field.) Accordingly, in the improved device, a period during which a signal charge stays in a vertical CCD is short, and hence the amount of smear charge which leaks in the vertical CCD, decreases. That is, a smear appearing on the display screen is reduced. However, the improved device includes the charge storage part, and therefore has a drawback that the area of the imaging chip is large.
In other conventional imaging devices capable of reading out signal charges of two photodiodes in adjacent rows independently of each other, the number of stages included in each vertical CCD is made twice as large as the number of rows in which photodiodes are arranged. Alternatively, as shown in FIG. 3, one vertical CCD of FIG. 1 is divided into two vertical CCD's, and signal charges from a pair of photodiodes in adjacent rows are transferred to one and the other of the two vertical CCD's, to be transferred independently of each other. The imaging device of FIG. 3 is disclosed in a Japanese utility model application specification (Laid-open No. sho 58-56458). In these imaging devices, however, fine patterning is required for fabricating an imaging chip, and a ratio of the light receiving area of each picture element to the whole surface area thereof is decreased, that is, the light sensitivity of the picture element may be reduced. Further, there arises a problem that the dynamic range of each vertical CCD may be reduced. Additionally, owing to the fine patterning, a region necessary for isolating a picture element from a vertical CCD is made narrow, and hence the amount of smear charge which leaks in the vertical CCD may be increased. Thus, a smear on the display screen may also be increased.